The present application is directed to acoustic-based devices, and more particularly, to acoustic resonators formed as Thin Film Bulk Acoustic Resonators (FBARs). It is to be appreciated, however, the following concepts may be implemented in other acoustic based devices.
FBARs are gaining increased use in handheld communication devices and are posed to replace larger bulk ceramic RF filters which are designed on the centimeter (cm) scale, as compared to FBARs, which are in the micrometer (μm) size range.
In particular, as handheld communication devices, such as cell phones, personal digital assistants, beepers, global positioning devices, hand-held digital music and/or video players, among others, become smaller, and include additional functionality, it becomes important to reduce the cost and size of the electronic components. This has led to a continuing need for smaller signal control elements such as the FBARs.
As shown in FIG. 1, in its simplest form, an FBAR 10 includes bottom and top electrodes 12, 14 separated by a piezoelectric (PZ) 16. The electrodes 12, 14 are energized by power source 18, such as an RF power supply.
FBAR 10 is supported (not shown) at its outer perimeter to permit movement of the piezoelectric. When FBAR 10 is energized by power source 18, an electric field is created between electrodes 12, 14, and piezoelectric 16 converts some of the electrical energy into mechanical energy in the form of mechanical waves. The waves propagate in the same direction as the electric field, and reflect off the electrode/air interface.
A resonant mode exists when the thickness of FBAR 10 is equivalent to an integer multiple of one-half of the acoustic wavelength. More particularly:
      d    =          n      ⁢                        λ          res                2              ,where n=an integer, λres=resonant wavelength, and d=stack thickness. When in the resonant mode, FBAR 10 can be employed as an electronic resonator.
Presently, a common procedure for manufacturing FBARs is through the use of deposition and micro-forming techniques employed for the fabrication of integrated circuits. More particularly, piezoelectric 16 may be formed by sputtering a material, such as aluminum nitride (AlN), which is commonly formed as a polycrystalline material, and which is a preferred material for existing FBAR production. Examples of FBARs using polycrystalline on a silicon substrate are disclosed in U.S. Pat. No. 5,587,620, issued Dec. 24, 1996, entitled “Tunable Thin Film Acoustic Resonators and Method for Making the Same,” by Ruby et al.; U.S. Pat. No. 6,060,818, issued May 9, 2000, entitled “SBAR Structures and Method of Fabrication of SBAR.FBAR Film Processing Techniques for the Manufacturing of SBAR/BAR Filters,” by Ruby et al.; and U.S. Pat. No. 6,710,681 B2, Issued Mar. 23, 2004, entitled “Thin Film Bulk Acoustic Resonator (FBAR) and Inductor on a Monolithic Substrate and Method of Fabricating the Same,” by Figueredo et al.
The described materials and the processes used to manufacture FBARs, are based on materials and techniques well known in the art for fabricating integrated circuits. The focus of existing FBAR manufacturing, which relies on processes and materials known to those working in the integrated circuit arena, has limited the search for other materials and/or processes which may be used to develop more efficient acoustic devices, including FBARs.